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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">In his discussion, the author (Sam) asks:<span style="color:#2C3E50;background:white"> “Why are there so few topologies? Why don't any of the existing topologies look much like filters?</span>”<span style="mso-spacerun:yes">
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif;color:#2C3E50;
background:white">The first question depends on what you mean by “few” and what constitutes a DIFFERENT topology.<span style="mso-spacerun:yes">
</span>If he means that most VCFs are “Ladders” (by including the modern equivalent of four cascaded first-order low-pass with feedback) or state-variable (S-V), he is correct in his observation.
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif;color:#2C3E50;
background:white">The second question depends on what a filter is SUPPOSED to look like. If he is thinking of a filter as a (fixed-parameter) “active filter”, perhaps VCVS, MFIG,
etc., where you get TWO poles for EACH op-amp, then topologies where you expend an op-amp for each pole (4-Pole LP) or three op-amps for two pole (S-V) look strange perhaps.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif;color:#2C3E50;
background:white">BOTH questions have answers that are understood in terms of the same design restraints – a limited number of PRACTICAL ways of achieving voltage-control.
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif;color:#2C3E50;
background:white">For example, if you started with a VCVS you have two floating resistors (not one end to ground or to virtual ground) and these would cost you two OTAs (and two
op-amps) for each such float: <o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><a href="http://electronotes.netfirms.com/AN23.PDF" id="LPlnk21659" class="OWAAutoLink" previewremoved="true">http://electronotes.netfirms.com/AN23.PDF</a><o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">In such a case, getting 2<sup>nd</sup> order with two OTAs to virtual ground and two integrating op-amps and a summer is a good deal.<span style="mso-spacerun:yes">
</span>We speak of such as a S-V-“filter” but it came out of an analog computing topology.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">But it is an explanation of engineers using methods/resources wisely.
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">Bernie</span></p>
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